If you need to do it by dynamic code you are boulding the parameters for sp_executesql incorrectly.

Short example:

DECLARE @SQLString nvarchar(500);

DECLARE @ParmDefinition nvarchar(500);

DECLARE @CountSQLQuery varchar(30);

SET @SQLString = N'SELECT @result = COUNT(*) FROM INFORMATION_SCHEMA.TABLES';

SET @ParmDefinition = N'@result varchar(30) OUTPUT';

EXECUTE sp_executesql @SQLString, @ParmDefinition, @result=@CountSQLQuery OUTPUT;

SELECT CAST(@CountSQLQuery as int);

Good luck!

/Markus